1. Field of the Invention
The invention relates to magnetic resonance imaging and, more particularly, to a method for producing an image of an object from a gradient echo pulse sequence produced in response to a non-selective composite hard RF pulse.
2. Description of Related Art
Magnetism results from the motion of electric charges such as electrons. Electrons can produce a magnetic field either by motion along a path or by virtue of their intrinsic spin. The particles that comprise the atomic nucleus, collectively called nucleons, also have spin and magnetic moment. Because both individual nucleons and nuclei have a charge distribution, rotation or spin of this charge produces a magnetic dipole whose value is called a magnetic moment. The numeric value of the magnetic moment determines the energies of the different orientations of a nucleus in an external magnetic field. The proton is positively charged and has a relatively large magnetic moment. Although neutral, the neutron also has a net magnetic moment. The neutron's magnetic moment is about two-thirds the value of the protons and points against the axis of spin. When in the nucleus, like nucleons align with their spins pointing against each other. This phenomenon is called pairing and is favored because it leads to a lower nuclear energy state. Therefore, only the unpaired, odd proton or neutron, or both, contribute their magnetic moment to the nucleus. As a consequence, only nuclei with odd numbers of protons or neutrons, or both, have a magnetic moment. The magnetic properties of nuclei become important when they are placed in external magnetic fields, as the nuclei will have a tendency to align with the external field.
Resonance occurs when an amount of energy equal to the difference of energy associated with the transition between states is absorbed or released. In the case of a magnetic moment of the nucleus, transitions between parallel or "up" and anti-parallel or "down" states can occur if the correct amount of energy is absorbed or released. Because the interaction is with a magnetic element, the necessary energy can be provided by a magnetic field. One way to obtain such a field is by utilizing electromagnetic radiations. To induce resonance, the frequency f of the electromagnetic radiation must be proportional to the local magnetic field H.sub.L. The particular proportionally constant which will induce resonance varies depending on the particular nucleus involved. The relationship between frequency and field is given by EQU f=(gamma)H.sub.L /2(pi) (1)
where (gamma) is the magnetogyric ratio of the nucleus.
When the nuclei, originally in equilibrium with the field, are irradiated at the resonant frequency, the nuclei can adopt the anti-parallel state. When they return to equilibrium, if the field is unchanged, they will radiate emissions of the same frequency. If between excitation and radiation the field strength is changed, they will radiate at a frequency corresponding to the new field value. The behavior of nuclei may be described by a net magnetization vector M which characterizes the system by disregarding the state of each nucleus and considers only the net collective effect. In a magnetic field, the magnetization vector points along the field, its length proportional to the number of nuclei in the sample and to the field strength and inversely proportional to temperature. The length and direction of this vector characterized the equilibrium magnetization of the sample; that is, the state that it will revert to after being disturbed if enough time is allowed to pass. This equilibrium magnetization is given by EQU (mu).sup.2 H/kT (2)
where:
(mu) is the nuclear magnetic moment; PA0 k is Boltzmann's constant; and PA0 T is the absolute temperature.
The vector can be disturbed from equilibrium by application of a second external magnetic field. If such a field is superimposed upon the first magnetic field, M will align with the new net field. As M moves to its new direction, energy stored in the nuclei of the sample is provided by the second field. When the superimposed field is removed, M returns to equilibrium and the nuclei release the stored energy to the environment, either as heat or RF energy. These two fields are called the transverse field and the longitudinal field, respectively. More specifically, the component of M that points along the main field is called the longitudinal magnetization (M.sub.L) and the orthogonal component is called the transverse magnetization (M.sub.T). If the transverse field is an RF field at the resonant frequency, M behaves as a top, so that as it deviates from the longitudinal axis, it precesses about it. If the main magnetic field is defined as being aligned along the z axis, then M.sub.T rotates in the x,y plane and M.sub.L is reduced from its equilibrium value. If M is rotated onto the x,y plane by a 90 degree RF pulse, M.sub.L is zero. Immediately after a RF irradiation, M.sub.L begins to grow again towards its equilibrium value M. The growth is exponential with a time constant T1 so that EQU M.sub.L =M[1-exp(-t/T1)] (3)
where t is the time since irradiation.
During this process, M.sub.T decays exponentially with a time constant T2, so that EQU M.sub.T =M.sub.TO exp(-t/T2) (4)
where M.sub.TO is the value of M.sub.T immediately after irradiation and t is the lapse time.
When a proton is aligned with the magnetic field, it gives off no signal. When a proton is perpendicular to the field, it gives off a maximum signal. The rate at which a proton realigns with the static field is called its "T1" or "T1 relaxation time". The T1 rlaxation time is also called spin-lattice or thermal relaxation time. The individual protons exchange fixed amounts of energy when they flip from the down to up alignment in the process of returning to equilibrium. This exchange can occur only at the resonant frequency. A molecule in the lattice surrounding the resonant nucleus appears as an oscillating electric and magnetic field with frequency that depends on its thermal velocity and mean free path. Since both vary over a broad range for any one temperature, of the whole ensemble of molecules only a small fraction provide the right oscillating fields. These then couple to the nucleus and allow the relaxation to occur. As temperature and molecular composition changes, so does the distribution of velocities and mean free paths, thus affecting T1.
When a group of protons precess in phase, the voxel gives off a maximum signal. When a group of protons precess out of the phase, the voxel gives off no signal. The rate at which the protons de-phase is called its "T2" or "T2 relaxation time". The T2 relaxation time is also called the spin-spin or transverse relaxation time. In a perfectly uniform magnetic field, all nuclei will resonate at exactly the same frequency. But if the field is even slightly inhomogeneous, nuclei resonate at slightly different frequencies. Although immediately after a RF irradiation they are all in phase, they soon lose coherence and the signal that is observed decays. Any loss of coherence shortens T2. The effects due to inhomogeneities and the external field produce a rapid decay characterized by the relaxation time T2.
Magnetic resonance has become an established method for producing an image of the interior of an object. Such method have numerous applications, particularly in medical diagnostic techniques such as the diagnosis of internal derangements of the knee. Most magnetic resonance techniques for knee imaging use a two dimensional (or "2D") acquisition with a spin-echo pulse sequence to provide T1, T2 and proton density weighted images of the knee in multiple planes, typically the sagittal (y-z) and coronal (x-z) planes. However, obtaining images in non-orthogonal planes is often advantageous for diagnosing internal derangements of the knee. For example, directly acquired oblique views of the anterior cruciate ligament and radial scans of the menisci have provided additional diagnostic information that was not available from the conventional views. However, to obtain images in a non-orthogonal plane, the use of two gradients rather than a single gradient are required to obtain a slice. Furthermore, oblique plane imaging of an object requires a corrective procedure after obtaining each gradient echo to keep the slices passing through the object being imaged.
As an alternative method for directly acquiring multiple image planes, a three dimensional (or "3D") acquisition with nearly isotropic resolution has been employed. See, for example, the publications to Harms and Muschler, "Three-dimensional MR Imaging of the Knee using Surface Coils", Journal of Computer Assisted Tomography; 10(5): 773-777 (1986) and C. Sherry et al., "Spinal MR imaging: Multiplanar Representation from a Single High Resolution 3D Acquisition", Journal of Computer Assisted Tomography; 11(5): 859-862 (1987). While the images produced by this method had high resolution and good T1-weighted contrast, the images also suffered from a long scan time and lack of fluid enhancement.
Variations of fast scan sequences such as the "gradient refocused acquisition in a steady state" (or "GRASS") imaging method have been used to reduce the overall scan time of a 3D exam. See, for example, the publications to Robert L. Tyrrell, "Fast Three-dimensional MR Imaging of the Knee: Comparison with Arthroscopy", Radiology; 166: 865-872 (1988); Charles E. Spritzer, et al., "MR Imaging of the Knee: Preliminary Results with a 3DFT GRASS Pulse Sequence", American Journal of Roentology; 150: 597-603 (1987); Allan M. Haggar, et al., "Meniscal Abnormalities of the Knee: 3DFT Fast-Scan GRASS MR Imaging", American Journal of Roentology; 150: 1341-1344 (1988). However, the images produced by these fast scan sequences are limited by reduced contrast. For example, the soft tissue anatomy around the knee was often lost due to poor contrast between fluid, muscle, and fat and the hyaline cartilage and fluid were often indistinguishable. As a result, the contrast available for diagnosis of ligamentous and mensical injuries is not comparable to the contrast available on conventional spin echo images. Furthermore, chemical shift and magnetic susceptibility effects are more severe, thereby further compromising image quality. Finally, the signal-to-noise (or "SNR") of such 3D methods was limited compared to 2D methods because of the very thin sections employed to get nearly isotropic 3D data sets.
Another fast 3D magnetic imaging resonance technique of interest recently is generally referred to as a fast, low angle (or "FLASH") imaging method. See, for example, the publications to Jens Frahm, et al., "Rapid Three-Dimensional MR Imaging Using the FLASH Technique", Journal of Computer Assisted Tomography, 10(2); 363-368 (1986) and A. Haase, et al., "FLASH Imaging. Rapid NMR Imaging Using Low Flip-Angle Pulses", Journal of Magnetic Resonance, 67, 258-266 (1986). The publication to Haase et al discloses a FLASH magnetic resonance imaging technique using a slice selective RF pulse with flip angles on the order of 15 degrees. The publication to Frahm et al discloses a FLASH magnetic resonance imaging technique having an image acquisition time of 4 minutes for a 3D data set of dimensions 128.times.128.times.128 pixels. Data was acquired using a non-composite, nonselective RF excitation pulse with a flip angle of the order of 15 degrees. A pair of perpendicular phase encoding gradients were generated in the presence of a negative read gradient and an observed magnetic resonance signal was detected in the form of a full symmetric field echo having an echo time of approximately 9 milliseconds which was generated by reversing the read gradient. Reconstruction of the 3D image was performed by conventional 3D Fourier transformation.
It is an object of this invention to provide a method for producing fast, high resolution MR images using three dimensional imaging techniques.
It is another object of this invention to provide a method for producing fast, high resolution gradient echo images having good T1 contrast, a high signal to noise ratio and minimized susceptibility artifacts.
It is yet another object of this invention to provide a method for producing fast, high resolution gradient echo images by minimizing echo times.